Among driving methods of a display device using self-luminous light emitting elements typified by organic light emitting diodes (OLEDs) (also referred to as organic EL elements, electro luminescence (EL) elements and the like), there are known a passive matrix method and an active matrix method. The former has a simple structure, but has a problem that a large and high-luminance display cannot be realized easily. Therefore, the active matrix method has been recently developed which controls a current flowing to a light emitting element with a thin film transistor (TFT) provided in a pixel circuit.
In the case of a display device using the active matrix method, a problem is recognized that currents flowing to light emitting elements vary due to the variation in the current characteristics of driving TFTs, leading to luminance variation. That is, driving TFTs for driving currents flowing to light emitting elements are employed in pixel circuits. When the characteristics of these driving TFTs vary, currents flowing to the light emitting elements also vary, leading to luminance variation. In view of this, various circuits for suppressing luminance variation are proposed in which currents flowing to light emitting elements do not vary even when the characteristics of driving TFTs in pixel circuits vary (see Patent Documents 1 to 4, for example).
[Patent Document 1]
Published Japanese Translation of PCT International Publication for Patent Application No. 2002-517806.
[Patent Document 2]
International Publication WO01/06484.
[Patent Document 3]
Published Japanese Translation of PCI International Publication for Patent Application No. 2002-514320.
[Patent Document 4]
International Publication WO02/39420.
Patent Documents 1 to 3 each discloses a circuit configuration for preventing variation in current values flowing to light emitting elements due to the variation in the characteristics of driving TFTs disposed in pixel circuits. This configuration is referred to as a current write type pixel or a current input type pixel. Patent Document 4 discloses a circuit configuration for suppressing variation in signal currents due to the variation of TFTs in a source driver circuit.
FIG. 6 shows a first exemplary configuration of a conventional active matrix display device disclosed in Patent Document 1. The pixel in FIG. 6 comprises a source signal line 601, first to third gate signal lines 602 to 604, a current supply line 605, TFTs 606 to 609, a storage capacitor 610, an EL element 611 and a current source for video signal current input 612.
Operations from signal current writing to light emission are described now with reference to FIG. 7. Reference numerals given to each portion are based upon those in FIG. 6. FIGS. 7A to 7C schematically show current flows. FIG. 7D shows a relationship of a current flow through each path when signal currents are written. FIG. 7E shows a voltage accumulated in the storage capacitor 610, namely a gate-source voltage of the TFT 608 when signal currents are written as well.
First, pulses are inputted to the first gate signal line 602 and the second gate signal line 603, thereby the TFTs 606 and 607 are turned ON. At this time, a current flow through the source signal line, namely a signal current is referred to as Idata.
Since the current Idata flows through the source signal line, current paths in the pixel branches into I1 and I2 as shown in FIG. 7A. FIG. 7D shows their relationship. It is needless to say that Idata=I1+I2 is satisfied.
At the moment at which the TFT 606 is turned ON, the storage capacitor 610 has not yet held charges, therefore, the TFT 608 is OFF. Accordingly, I2=0 and Idata=I1 are satisfied. That is, a current only flows in accordance with the charge accumulation in the storage capacitor 610.
After that, charges are gradually accumulated in the storage capacitor 610, and a potential difference starts to be generated between opposite electrodes thereof (FIG. 7E). When the potential difference between the opposite electrodes reaches Vth (point A in FIG. 7E), the TFT 608 is turned ON, generating I2. Since Idata=I1+I2 is satisfied as set forth above, I1 decreases gradually, however, current still flows, and thus the storage capacitor further accumulates charges.
The storage capacitor 610 keeps on accumulating charges until the potential difference between the opposite electrodes thereof, namely the gate-source voltage of the TFT 608 reaches a voltage (VGS) required for the TFT 608 to flow current Idata. When the charge accumulation terminates point B in FIG. 7E) in the meantime, the current I1 does not flow any more, and a current corresponding to VGS at this time flows into the TFT 608, thereby Idata=I2 is satisfied (FIG. 7B). Accordingly, a steady state is obtained. The signal writing operation is completed in this manner. At the end, selection of the first gate signal line 602 and the second gate signal line 603 terminates, and thus the TFTs 606 and 607 are turned OFF.
Subsequently, the operation proceeds to a light emitting operation. A pulse is inputted to the third gate signal line 604, thereby the TFT 609 is turned ON. The storage capacitor 610 holds the previously written VGS, therefore, the TFT 608 is ON and the current Idata flows therethrough from the current supply line 605. Accordingly, the EL element 611 emits light. At this time, if the TFT 608 is set to operate in the saturation region, Idata can flow constantly even when the source-drain voltage of the TFT 608 changes.
In this manner, the operation of outputting a set current is hereinafter referred to as an output operation. Such current write type pixel has an advantage that even in the case where the TFT 608 has variation in the characteristics and the like, the storage capacitor 610 can hold the gate-source voltage which is required to flow the current Idata, therefore, a predetermined current can be supplied to an EL element accurately, which makes it possible to suppress luminance variation due to the variation in the characteristics of TFTs.
The aforementioned examples are related to a technique of correcting variation in currents due to the variation of driving TFTs in pixel circuits. The same problem occurs in a source driver circuit. Patent Document 4 discloses a circuit configuration for preventing variation in signal currents due to the manufacturing tolerance of TFTs in a source driver circuit. [Patent Document 5] Japanese Patent Laid-Open No. 2003-66908.
Patent Document 5 discloses a configuration comprising a voltage source as well as a current source for controlling gray scales, wherein a charge of a floating capacitor is instantaneously changed by the voltage source at the beginning of the row selection period by using a power source switching means for switching the two power sources inputted to a source signal line, and gray scales are displayed by a current source 10 to obtain a predetermined luminance.